 
 (defn grow
   "recursively generate trees given 
-   input trees and model. continue recursively
+   input trees. continue recursively
    until no futher expansion is 
    possible."
   [trees]

    (let [parent-rule (first parent-rules)
          phrases-with-phrasal-head #(map (fn [child]
                                            (u/assoc-in parent-rule [:head] child))
                                          (:grammar model))
          phrases-with-lexical-heads #(map (fn [child]
                                             (u/assoc-in parent-rule [:head] child))
                                           (get-lexemes (unify

                        result))
(def ^:dynamic truncate? false)
(def ^:dynamic println? false)
(def ^:dynamic default-fn (fn [x] [x]))
(def ^:dynamic model)
(def ^:dynamic morph-ps)
(def ^:dynamic index-fn)
(def ^:dynamic grammar)

(declare assoc-children)
(declare frontier)
